This invention relates to recycling, and more particularly to a process and an apparatus for recycling of PET constituents from comminuted plastic beverage bottles.
In PET washing and treatment installations which are known in practice, PET flakes (ground pieces of plastics, of size about 5 to 10 mm) are treated in washing centrifuges or in stage treatment cleaning devices at high speeds of rotation and under considerable friction, with the addition of relatively small amounts of water. Due to the very high power which is used for a relatively short treatment time, the flakes are subjected to severe mechanical damage, and small pieces together with cracked and split flakes are produced, which can cause problems in further stages of the treatment process. These small pieces have to be removed, which is costly and which reduces the yield. Other disadvantages are that the surface of the flakes are only cleaned unsatisfactorily, namely to a shallow depth, bonding adhesives cannot reliably be removed, and extraneous substances which have diffused into the surface cannot satisfactorily be dissolved out or dissolved away. The extraneous substances can only satisfactorily be removed in further stages of the treatment process, in a different manner which involves significant additional cost.
In a process which is known from EP 0 304 667 A, fine material and paper is separated from the comminuted material in an air classifier, before dust and the like are removed from the flakes, together with the removal of label residues, in a multi-stage fraction washer. Passage through a washing vessel and a suspension vessel is followed by a further separation stage by means of a hydrocyclone arrangement in order to separate what is by far the bulk of the PET-flakes from the remaining fractions. Relatively heavy metal particles which are still present are further treated and are then optionally sorted. This process is extraordinarily costly on an industrial scale, and although it can result in a high degree of separation the final quality of the flakes nevertheless is unsatisfactory.
In a process which is known from U.S. Pat. No. 5,580,905A for the recycling of plastics waste which contains PET materials amongst others, a slurry is produced using a caustic soda solution without any mechanical washing treatment, and is heated in an environment of low oxygen content to the distillation temperature of the polyols, in order to obtain polyester salts.
In a process which is known from DE 195 45 357A for the re-utilization of comminuted plastics materials, water-washing and sink-float separation, or hydrocyclone separation, are employed. During washing and/or separation, a hydrocarbon-based organic washing medium which has a defined density is used. For example, a mineral oil fraction and/or a raffinate fraction therefrom, optionally with polar solvents can be used.
It is known in principle that PET bottles can be cleaned at an elevated temperature for 20 minutes, using a cleaning solution which contains caustic soda. Sensitivity of PET bottles (stress cracking and shrinkage phenomena), however, means that restricted upper limits have to be adhered to with regards the concentration of the cleaning solution, the treatment temperature, and particularly, with regards the treatment time. These constraints result in a degree of purity which would be too low for the recycling of PET flakes for the production of new bottles.
The underlying object of the present invention is to provide a process of the type noted at the outset, and an apparatus for carrying out the process, for obtaining flakes of high quality, which can even be used for the manufacture of new bottles from said flakes, can be obtained without costly subsequent processing steps.
A method for recycling of PET constituents from plastic beverage bottles includes comminuting the plastic beverage bottles to form a mixture of particles containing PET flakes. Washing the PET flakes follows, which includes simultaneously treating the PET flakes hydraulically and mechanically in at least one washer with a cleaning solution including caustic soda at a temperature exceeding 70xc2x0 C. and for more than 20 minutes.
A system for recycling PET flakes from beverage bottles having been comminuted to form a mixture of particles includes a treatment section adapted to treating the mixture including at least one washer. The washer includes a stirrer, at least one float-sink separator, and mechanical and hydraulic treatment devices adapted to treating the PET flakes. A feeder device feeds a cleaning solution including caustic soda to the washer. A heating device heats the cleaning solution including caustic soda.
Because the flakes are treated for more than 20 minutes, at least in the washer, with a cleaning solution which contains caustic soda at an elevated temperature above 70xc2x0 C., and are thus also processed mechanically and hydraulically at the same time, even the label adhesives which are currently on the market can be removed from the flakes, so that only a low level of subsequent treatment is required, if at all, in order to be able to process the flakes directly. As a result, the surface quality of the flakes is outstanding. The cost of washing the flakes is in fact relatively high, both with regards the process and with regards the apparatus, but this is more than offset by the high final quality and surface quality of the flakes, and the omission of costly additional processing stages constitutes a definite advantage over conventional treatment processes, because particular emphasis is placed on the initial phase of the treatment process.
The apparatus is designed from the outset for an extremely long treatment time, for high treatment temperatures, and for the use of a cleaning solution which contains caustic soda, so as to be able to subject the flakes to mechanical, hydraulic, and intensive thermal treatment.
Particularly satisfactory results are obtained by employing a treatment time of about 30 minutes at 80 to 90xc2x0 C. Higher temperatures can also be used, however, particularly if reduced pressure is employed.
In order to utilize process technology parameters and knowledge from the cleaning of PET bottles, a treatment time is employed which is prolonged by up to 50 per cent compared with the treatment time which is employed for the cleaning of PET bottles, an elevated treatment temperature is employed compared with the treatment temperature which is possible for the cleaning of PET bottles, and a higher concentration of cleaning solution is also optionally employed.
The surface of the flakes is cleaned very intensively due to the mechanical stirring and to the simultaneous subjection of the flakes to hydraulic high-pressure spraying.
It is advantageous if sieving and/or filtration is effected in the washer, or at least in a subsequent processing stage which is coupled to the washer in the process. Parts of labels, fibres and pieces of foil, as well as particles of dirt, can thereby be separated from the flakes and can be removed from the washing liquor.
At least one additive can be added to the cleaning solution in order to intensify the cleaning effect.
Due to the further treatment of the flakes in a sink-float separator with the cleaning solution which contains caustic soda, the surface quality of the flakes which is achieved using a long treatment time is so good that the flakes require no appreciable further processing.
Due to the subsequent intensive washing in an intensive washer, likewise with the use of the cleaning solution which contains caustic soda, even tenacious iridescent stains and coatings are reliably removed.
The treatment in the sink-float separator or in the intensive washer is advantageously carried out with recirculation or partial recirculation, in order to achieve a sufficiently long treatment time and an increased throughput with a more intensive cleaning effect.
Following the intensive washing stage, the cleaning solution which contains caustic soda is separated and recycled, so that the cleaning solution can be reused repeatedly with relatively small losses.
In this respect, it is advantageous if the concentration of the cleaning solution is continuously monitored and is adjusted by metered additions of at least a caustic soda solution.
The elevated treatment temperature is advantageously achieved by heating the washer.
The washer advantageously contains sieve plates and automatic filters in order to detach and reject label constituents, fibres, pieces of foil and particles of dirt.
In order to maintain a treatment temperature between about 80 and 90xc2x0 C., even under conditions of high throughput or when the washer is of large capacity, it is advantageous if at least one heat exchanger and/or a direct heater device is provided in the heater device, wherein the temperature can be adjusted and maintained in a particularly sensitive manner by employing an electro-pneumatic control system.
The use of a high-pressure pump and nozzle pipes results in the high-pressure cleaning of the entire surface of the flakes. The sieve plates, filters and nozzle pipes are advantageously fixedly disposed in relation to a plurality of stirrer stages, so that the mixture in the washer is also uniformly and intensively agitated.
Due to the coupling of the sink/float separator to the washer in the process, the hot cleaning solution which contains caustic soda is also used in the separator.
In a downstream intensive washer through which the flakes and a partial stream of the cleaning solution are conveyed, e.g. by a pump, an additional surface cleaning effect is ensured in which even tenacious iridescent stains and coatings are reliably removed. A recycle section can be provided between the intensive washer and the sink-float separator, in order to create at least partial recirculation and to achieve the requisite hold-up time at high throughputs without employing vessels of extremely large volume.
Following the intensive washer, the cleaning solution which contains caustic soda is separated in a separator and is recycled to the preceding process stages. For this purpose, a supply branch can be provided which leads to the washer and which advantageously comprises at least one supply pump.
At least one caustic soda measuring and metering unit should be present in the feeder device. This measuring and metering unit is connected to the supply branch or is connected thereto in some situations, but can optionally also be connected to the sink-float separator so as to be able to adjust the concentration in the sink-float separator or even to make individual adjustments therein.
So as not to adversely affect the treatment temperature, particularly in the washer, during the feeding of caustic soda and/or additives, a preheater device should be provided in the metering unit.
After the cleaning solution has been separated, it is advantageously passed through a neutralizer which is supplied with fresh water and with an acid, e.g. phosphoric acid or CO2, with corresponding metering, and the PET material is neutralized. Rinsing with fresh water can even be carried out here for cleaning purposes.
The sink-float separator is advantageously also supplied with fresh water, in order to deliberately set a concentration which is different from that in the washer, or specifically to compensate for losses.
In order to separate at least part of the heavy material which may be contained in the mixture, a heavy material separator should be disposed upstream of the washer.
The washer is advantageously designed with an uptake and throughput capacity for a treatment time of more than 20 minutes, preferably about 30 minutes.